By far the easiest of the three engines to use and can build games for PC, Mac, and mobile platforms. Unity comes with its own terrain system however this isn't available in the Unity iPhone/Android editor. This is simply because mobile platforms can't (yet) render the hundreds of thousands of polygons that typically constitute landscapes on their larger desktop counterparts.

In spite of this limitation, Unity iPhone/Android has been partly addressed by including a simple terrain system for mobiles, GROME still has this well in hand through its use of Mesh layers.

Mesh layers are 3D models based on the terrain heightmap exported in Collada (.dae) format. We can export our terrain with enough detail for a target mobile device or for PlayStation, XBOX, and PC. We can do this for multiple levels of detail (LOD). For desktops we can simply use the heightmap from GROME and load it into the Unity editor so we can use all of the native Unity terrain tools for painting and vegetation. This...